Repositionable mask for ultrasonic inspection

ABSTRACT

Methods and apparatus are provided for protecting an acoustic liner having a perforated facesheet from water leakage during a through-transmission ultrasonic (TTU) inspection using an ultrasonic squirter system. The apparatus comprises a repositionable mask that covers the openings in the perforated facesheet. The repositionable mask is comprised of a pressure sensitive adhesive that adheres to the surface of the perforated facesheet. The repositionable mask can be removed after completing a TTU inspection and can be reused for subsequent TTU tests with minimal adverse effects on the mask or on the facesheet. For ease in handling, the repositionable mask may be configured with a semi-rigid backing carrier.

TECHNICAL FIELD

The present invention generally relates to through-transmissionultrasonic (TTU) inspection, and more particularly relates to arepositionable mask that is used to protect the article being TTUinspected.

BACKGROUND

Acoustic liners are generally used to suppress sound propagation from anoisy source. In the aerospace industry, for example, acoustic linersare often used to reduce the noise emanating from an aircraft engine andfan assembly. Typically, the aircraft engine and fan are housed within anacelle enclosure, and acoustic liners are generally integrated withinthe nacelle structure. While acoustic liners can be fabricated frommetal or composite materials, composites offer a number of advantages,such as weight reduction and improved fatigue resistance in a high sonicenvironment, among others. Composite acoustic liners are typicallyfabricated in a sandwich-type configuration, with a solid backsheet, ahoneycomb sound-absorbing core middle section, and a perforatedfacesheet.

Composite acoustic liners for this type of noise suppression applicationare generally inspected by an ultrasonic technique, such as a“through-transmission ultrasonic” (TTU) process. One type of TTUinspection procedure, known as an ultrasonic squirter system, utilizeswater streams that are directed against the outside surfaces of thearticle being inspected, in order to carry the ultrasonic inspectionsignal from an ultrasonic transmitter through the test article to anultrasonic receiver. In the case of a composite acoustic liner TTUinspection, the perforated facesheet on the acoustic liner is generallymasked with some type of taping material to prevent water from enteringthe honeycomb structure through the facesheet perforations, since thepresence of water in the honeycomb structure can cause problems in thesubsequent finish and assembly operations. After completing a TTUinspection, the masking material is typically removed manually, anddiscarded. For applications involving relatively large test articles,such as aircraft nacelle acoustic liners, the manual taping and taperemoval processes can be labor-intensive, and may also result in thedeposit of masking material residue on the acoustic liner surface,leading to a possible degradation in performance of the acoustic liner.

Accordingly, it is desirable to provide a mask for TTU squirter systeminspection of an acoustic liner that is relatively easy to apply andrelatively easy to remove, and that does not deposit residue on thesurface being masked. In addition, it is desirable to provide a maskthat can be repositioned (i.e., reused) for subsequent ultrasonictesting. Furthermore, other desirable features and characteristics ofthe present invention will become apparent from the subsequent detaileddescription and the appended claims, taken in conjunction with theaccompanying drawings and the foregoing technical field and background.

BRIEF SUMMARY

According to various exemplary embodiments, devices and methods areprovided for protecting one or more surfaces of an article from waterwhile the article is undergoing squirter system through-transmissionultrasonic (TTU) inspection. One embodiment comprises a repositionablemask in the form of a pressure sensitive adhesive layer configured toadhere to the one or more surfaces of the article. The pressuresensitive adhesive layer is also configured to be removable from the oneor more surfaces of the article after completion of squirter system TTUinspection. Moreover, the removed pressure sensitive adhesive layer canbe reused for protecting other article surfaces during subsequentsquirter system TTU inspections.

An exemplary embodiment of the repositionable mask is typicallyfabricated from an ethylene vinyl acetate copolymer with approximately50% vinyl acetate content to provide the desired adhesioncharacteristics. An alternate embodiment of the repositionable mask canbe configured with a backing material carrier integrated with thepressure sensitive adhesive layer. The backing material carrier istypically configured as a semi-rigid pre-contoured carrier, and isgenerally fabricated from thermoformed acrylic or from thermoformedpolycarbonate.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a simplified illustration of an exemplary aircraft engine podconfiguration;

FIG. 2 is an illustration of an exemplary acoustic liner for an aircraftengine nacelle;

FIG. 3 is a cross-sectional illustration of an exemplary acoustic linerhaving a perforated face sheet;

FIG. 4 is a cross-sectional illustration of an exemplary embodiment ofan acoustic liner with a repositionable mask for protecting theperforated face sheet; and

FIG. 5 is an illustration of an exemplary embodiment of an ultrasonicsquirter system TTU inspection arrangement.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary or the following detailed description.

Various embodiments of the present invention pertain to the area ofthrough-transmission ultrasonic (TTU) squirter system inspection ofarticles susceptible to water intrusion, such as acoustic liners withperforated facesheets and honeycomb interiors. To protect this type ofarticle during a TTU squirter system inspection, a mask is typicallyapplied to the water-susceptible surface(s) to prevent water dispensedby the ultrasonic squirter system from entering the article interior(honeycomb section). The exemplary mask to be described below istypically fabricated from a pressure sensitive adhesive that isrelatively easy to apply and also relatively easy to remove. Theexemplary mask is designated herein as a “repositionable” mask becauseit can generally be removed intact from a test article surface, and thenreused for subsequent TTU inspections.

As noted in the Background, acoustic liners are commonly used tosuppress the noise emanating from an aircraft engine and fan assembly,as well as other associated noise-generating components. A simplifiedillustration of an aircraft engine pod configuration 100 is shown inFIG. 1. In this example, a nacelle 102 is attached via a strut 104 to awing 106. Nacelle 102 typically encloses an engine 108, an engine fan110, and an acoustic liner arrangement 112A, 112B. Depending on the typeand location of noise suppression desired, an acoustic liner arrangementcan be configured in various forms, and is typically integrated into thestructure of nacelle 102. For example, the acoustic liner configurationshown in FIG. 1 includes an outer duct wall 112A with a perforatedfacesheet on the “inner” surface facing engine 108 and fan 110, and aninner duct wall 112B surrounding engine 108 with a perforated facesheeton the “outer” surface facing outer duct wall 112A. This type ofacoustic liner arrangement is typically designed to suppress noise inarea A that is largely generated by engine 108 and air flow from fan110, which may be running at a speed in the approximate range of sixthousand (6,000) RPM.

An exemplary configuration of an acoustic liner 200 (e.g., outer ductwall 112A in FIG. 1) is illustrated in the isometric and end views ofFIG. 2. In this configuration, acoustic liner 200 is fabricated in twoparts, 200A and 200B, which, for example, can be assembled around anengine and fan assembly such as engine 108 and cooling fan 110. As notedabove, acoustic liner 200 is typically oriented with an inner (i.e.,concave) perforated facesheet to suppress engine and fan noise in areaA. It will be appreciated that other acoustic liner configurations(e.g., inner duct wall 112B in FIG. 1) may have outer (i.e., convex)perforated facesheets, depending on the application configuration.Moreover, it will also be appreciated that the shape and size of anacoustic liner can take many different forms, as determined by aparticular noise suppression application. The exemplary configuration ofacoustic liner 200, as depicted in FIG. 2, is used herein as merely oneexample of a convenient shape to aid in the description of arepositionable mask.

A typical cross-section of acoustic liner part 200A is illustrated insimplified form in FIG. 3. In this example, an outer layer 302 is shownas a solid backsheet covering the outer surface of an interior section304, which is typically fabricated in a honeycomb configuration forsound absorption. An exemplary inner layer 306 represents a perforatedfacesheet covering the inner surface of interior honeycomb section 304.A typical perforated facesheet configuration might include a number ofholes of approximately 0.04 inch in diameter spaced approximately 0.1inch apart, such that about 5% to 15% of the total surface of theperforated facesheet is open.

As noted above, acoustic liners are commonly inspected with TTU squirtersystems, as will be described more fully below. However, if water from aTTU squirter system enters an acoustic liner honeycomb structure througha perforated facesheet, significant processing problems can occur duringthe subsequent finishing operations. For example, the paints and surfacefilling compounds typically used can be fouled from water vapor evolvingfrom the structure during baking operations. To avoid this type ofproblem, the entire perforated area of the acoustic liner assembly istypically masked to prevent water ingression. The masking material isgenerally in intimate contact with the skin surface to allow theultrasonic inspection signal to pass through. A conventional maskingmaterial (e.g., an adhesive coated tape material) is typically appliedin a laborious manual process, in order to avoid the entrapment of airor wrinkles during the application process. The presence of voids ordisbands in the mask-to-skin interface may cause interference withultrasonic transmissions through the acoustic liner, and may lead to themisinterpretation of data.

Due to the typical compound contours and possible surface imperfectionsof acoustic liners, the masking material adhesive is generally selectedwith relatively aggressive tack characteristics, so that the mask canremain fully conforming to the liner surface. Once the mask is adheredto a liner surface (i.e., a perforated facesheet), it is generallydesirable for the mask to maintain full contact with the liner surfaceduring TTU inspection and other related procedures that may range intime to approximately 48 hours. It is also generally desirable that themask adhesion characteristics are not adversely affected by seasonalroom temperature changes within an inspection environment.

Another desirable property for a mask adhesive is relative ease ofremoval from a liner surface after the completion of a TTU inspection.While conventional taping materials can generally be removed afterinspection, the taping process can be labor-intensive andtime-consuming, especially for relatively large test articles such asaircraft acoustic liners. Moreover, residue from the taping materialsoften remains on the liner surface after removal, further complicatingthe process. In addition, conventional taping materials are routinelydiscarded after one application and removal, thereby representing aproduction cost consideration.

Because of the disadvantageous characteristics of conventional maskingtape materials as described above, a different approach to masking aperforated facesheet is disclosed herein that replaces the conventionalmulti-strip tape procedure with a single-piece mask material. Accordingto an exemplary embodiment of acoustic liner portion 200A with asingle-piece mask 400, as shown in FIG. 4, a single sheet of pressuresensitive adhesive (PSA) is formed into mask 400 with a size and shapeto cover the holes in perforated facesheet 306 of acoustic liner 200A.PSA mask 400 can be cast from a hot-melt adhesive or can be fabricatedby extrusion. One example of a suitable hot-melt adhesive is an ethylenevinyl acetate (EVA) copolymer with approximately 50% vinyl acetate (VA)content. Other types or combinations of PSAs may also be used, dependingon the adhesive characteristics (tack level) desired. The tack level ofexemplary EVA mask 400 can be adjusted in order to provide sufficientadhering strength in combination with adequate removal characteristics.For example, the tack level can be adjusted by modifying the molecularweight or the VA content of the material in mask 400. The properties ofmask 400 material can also be adjusted by compounding the material withfillers, or by blending the material with other polymers.

In general, an EVA copolymer with approximately 50% VA content remainspermanently tacky at room temperature without the use of solvents.Moreover, since EVA is a polymer, the tack level of the material isgenerally unaffected by material aging or by typical changes in roomtemperature. In addition, an EVA mask can generally be removed from afacesheet surface by peeling, and without leaving a residue.Furthermore, EVA masks do not typically contaminate the masked surface,and EVA masks generally do not exude residues upon aging.

Referring again to FIG. 4, an exemplary embodiment of EVA mask 400 isadhered to perforated facesheet 306 on acoustic liner 200A. As will bedescribed below, exemplary EVA mask 400 is typically configured toprotect honeycomb structure 304 from water leakage through theperforations in facesheet 306 during a TTU squirter system inspection.In certain applications, mask 400 may be mounted within a contouredframe (not shown), to minimize sagging or stretching of the maskingmaterial during application and removal. When a frame is employed, thedesign of the frame should be configured to avoid interference with theultrasonic transmission of a TTU inspection process.

In an alternative exemplary embodiment, mask 400 may be constructed witha backing 402, herein designated as a “carrier” for ease in handling,and shown in dashed line form in FIG. 4. In this embodiment, a sheet ofEVA PSA (400) having a thickness in the approximate range of about 0.03to 0.08 inch is typically applied to a thermoformed acrylic orpolycarbonate carrier (402) to form a mask/carrier combination prior toapplication to facesheet 306. The overall thickness of mask 400 andcarrier 402 is typically limited (e.g., to an approximate maximumthickness of 0.14 inch) in order to minimize interference with theultrasonic signal used in TTU inspections. A suitable carrier 402 forthis type of application is generally configured as a semi-rigidpre-contoured backing, with sufficient strength and compliance to permitassociated mask 400 to be peeled from facesheet 306 after completion ofthe TTU inspection. As such, the bond strength of mask 400 to carrier402 is typically greater than the bond strength of mask 400 to facesheet306. These relative bond strengths can be controlled through thejudicious selection of carrier material and bonding characteristics ofthe mask.

A typical TTU inspection arrangement 500 is depicted in FIG. 5 for anacoustic liner part 501 (similar in construction to liner 200A) and aprotective mask 503 (similar in construction to mask 400). A squirtersystem transmitter 502 generates an ultrasonic beam 505 along a waterstream 504 that impinges on the surface of mask 503 at a point C. Asnoted previously, mask 503 prevents moisture from water stream 504 fromentering the internal honeycomb section of acoustic liner part 501 viathe holes in the perforated facesheet of acoustic liner part 501 (notshown). Simultaneously, a squirter system receiver 506 generates a waterstream 508 that impinges on the solid backsheet of acoustic liner part501 in precise alignment with water stream 504. Ultrasonic beam 505passes through mask 503 and acoustic liner part 501, and continues alongwater stream 508 to receiver 506, where it is converted to output data.Typically, an ultrasonic squirter system acquires data from a universeof scanned points across the body of a test part (501 in this example)and then processes the data to determine the inspection results. Thistype of system is commercially available from sources such as BoeingAutomated Systems in St. Louis, Mo.

Accordingly, the shortcomings of the prior art have been overcome byproviding an improved mask for TTU squirter system inspectionapplications. Exemplary embodiments of a repositionable mask aredisclosed herein that protect an acoustic liner with a perforatedfacesheet from the water streams typically used in TTU squirter systeminspections. The exemplary repositionable mask is generally fabricatedfrom an EVA material with adhesion properties that enable the mask toadhere intimately to the liner surface to be protected, and also to beremovable from the liner surface with minimal adverse effects on eitherthe mask or the liner. As such, the mask can be repositioned (reused) onother liners for subsequent testing.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing the exemplary embodiment or exemplary embodiments. Itshould be understood that various changes can be made in the functionand arrangement of elements without departing from the scope of theinvention as set forth in the appended claims and the legal equivalentsthereof.

1. A repositionable mask configured to protect one or more surfaces ofan article from water while the article is undergoing squirter systemthrough-transmission ultrasonic (TTU) inspection, comprising: a pressuresensitive adhesive layer configured to adhere to the one or moresurfaces of the article, wherein the pressure sensitive adhesive layeris removable from the one or more surfaces of the article aftercompletion of squirter system TTU inspection, and wherein the removedpressure sensitive adhesive layer is reuseable for subsequent squirtersystem TTU inspections.
 2. The repositionable mask of claim 1 whereinthe pressure sensitive adhesive layer is substantially transparent toultrasonic signals used in the TTU inspection.
 3. The repositionablemask of claim 1 wherein the tack level of the pressure sensitiveadhesive layer is substantially constant in a room temperatureenvironment.
 4. The repositionable mask of claim 1 wherein the tacklevel of the pressure sensitive adhesive layer is substantially constantwith age.
 5. The repositionable mask of claim 1 wherein the pressuresensitive adhesive layer is substantially in full and intimate contactwith the one or more surfaces of the article.
 6. The repositionable maskof claim 1 wherein the pressure sensitive adhesive layer adheres to theone or more surfaces of the article for a time period of up toapproximately 48 hours.
 7. The repositionable mask of claim 1 whereinthe pressure sensitive adhesive layer is non-residue producing.
 8. Therepositionable mask of claim 1 wherein the pressure sensitive adhesivelayer is removed by peeling.
 9. The repositionable mask of claim 1wherein the pressure sensitive adhesive layer is fabricated from anethylene vinyl acetate copolymer.
 10. The repositionable mask of claim 9wherein the ethylene vinyl acetate copolymer includes substantially 50%vinyl acetate content.
 11. The repositionable mask of claim 1 furthercomprising a backing material carrier integrated with the pressuresensitive adhesive layer.
 12. The repositionable mask of claim 11wherein the backing material carrier is configured as a semi-rigidpre-contoured carrier.
 13. The repositionable mask of claim 12 whereinthe semi-rigid pre-contoured carrier is fabricated from thermoformedacrylic.
 14. The repositionable mask of claim 12 wherein the semi-rigidpre-contoured carrier is fabricated from thermoformed polycarbonate. 15.The repositionable mask of claim 11 wherein the bond strength of thebacking material carrier-to-the pressure sensitive adhesive layer isgreater than the bond strength of the pressure sensitive adhesivelayer-to-the one or more surfaces of the article.
 16. The repositionablemask of claim 1 wherein the pressure sensitive adhesive layer isfabricated by extrusion.
 17. The repositionable mask of claim 1 whereinthe pressure sensitive adhesive layer is fabricated by casting.